This invention relates to a method and arrangement for indicating multipath reception in a high frequency receiver receiving frequency modulated mono and/or stereophonic signals. In such a receiver, a frequency discriminator has an output from which, because of modulation distortion resulting from multipath reception, a signal indicating such multipath reception may be derived.
A known solution to this problem is based on phase modulation resulting from multipath reception. Such multipath reception causes a phase variation which results in a greater low frequency output voltage from the ratio detector then does the maximum frequency variation of 75kHz present in the transmitted signal.
However, in the above-mentioned type of arrangement, phase modulation resulting from pulse interference and through remainders of AM signal which are not sufficiently suppressed when the input voltages are small, are also indicated. This indication results for all frequencies which lie within the transmission band (0 to at least 53 kHz).
Another known method utilizes the amplitude modulation resulting from multipath reception. In this method, care must be taken that the synchronous characteristic amplitude modulation resulting from damping in the intermediate frequency amplifier during the transmission of frequency modulated oscillations does not enter into the indication. This means that the intermediate frequency amplifier must have a constant amplification at least within the region of the modulation spectrum.
It is a disadvantage of the above-explained method that AM interference is indicated which is not caused by multipath reception. Furthermore, if the indicator is to indicate the degree of modulation resulting from multipath amplitude modulation, a variable gain amplifier must precede the indicator. This variable gain amplifier must furnish a substantially constant output voltage for rectification of input voltages ranging from the smallest to the largest possible VHF antenna voltage. In simpler indicating circuits which do not indicate the intensity of the multipath amplitude modulation, the low frequency voltage required for the indicator is generally derived by rectification from each stage of the intermediate frequency amplifier. The resulting low frequency voltages are then furnished to the indicator by way of an adder. Thus the indicator is effective over the whole range of antenna input voltages.